A number of different vibratory devices already exist for industrial applications such as parts feeding. Perhaps the most common industrial vibratory device is the bowl feeder, which uses vibration to advance parts up a helical track in the interior of a bowl while wipers and cutouts reorient or reject incorrectly oriented parts. The more recent Sony APOS parts orienting system uses a tray of holes approximately shaped like the negative of the part. The vibratory driving is co-designed with the hole shape to capture parts that are correctly oriented in the hole but to eject parts that are incorrectly oriented in the hole. Parts not captured in holes wash over the tray and fall into a recirculation bin, eventually to be lifted back to the top of the tray and dumped over it again, until a full tray of parts is obtained.
U.S. Pat. No. 6,189,677 describes a device, initially developed for chocolates packaging, that uses a planar array of many vibratory cells to create a flexible conveyor. Each cell vibrates vertically while moving in small circles in the horizontal plane. The phasing of these motions determines the net force applied to parts in the cell, and virtual force fields can be designed by setting these phases such that the force directions generally vary among the cells. This system is related to research in creating planar arrays of “motion pixels” for distributed manipulation. These motion pixels may consist of rolling wheels, individual vibrating plates, vibrating MEMS elements, or air jets. These two-dimensional arrays of motion pixels create force fields to manipulate parts resting on top of the array. While these devices offer great flexibility, they require a large number of individually-controllable actuators.
Other physical effects have been explored for planar manipulation using fewer actuators, such as air flow with a small number of sinks and vertical vibration of a flexible plate. Nodes in a flexible vibrating plate can be visualized via classical Chladni patterns of granular materials on the plate.
The possibility of such systems has inspired the study of planar force fields for eliminating uncertainty in the configuration of a two-dimensional planar part. Force fields are integrated over the area of the part, and the part is at equilibrium when the resultant forces and torques are zero. For a part with no rotational symmetry, force fields have been derived that result in a unique globally-attractive part equilibrium.
Reznik and Canny have shown that vibratory motions of a flat horizontal part support plate, using only rotations and translations in a horizontal plane, can generate frictional force fields on the surface of the plate. These can be used to move parts on the plate on a variety of different trajectories as described by Reznik and Canny in “A flat rigid plate is a universal planar manipulator,” IEEE International Conference on Robotics and Automation, pages 1471-1477, 1998; “C'mon part, do the local motion!,” IEEE International Conference on Robotics and Automation, pages 2235-2242, 2001; and “Building a universal manipulator,” Distributed Manipulation, pages 147-171, Kluwer Academic Publishers, 2000.
As a result of the in-the-horizontal-plane nature of the plate motions, the force fields of Reznik and Canny are restricted to be divergence-free, however. That is, for any area of the plate, the force field flow into the area equals the flow out of the area. This precludes the possibility of sources and sinks in the field, and therefore these fields cannot be used for sensorless manipulation.